Epidemiological observations reveal a high prevalence of adverse cardiovascular events during early morning hours and during colder seasons of the year. No studies have been found which investigated the interactive effects of time of day and cold environments on hemodynamic or hemostatic factors that have been associated with cardiovascular mortality. The objective of this project is to investigate the effects of ambient temperature on hemodynamic (cardiac output, total peripheral resistance, blood pressure and plasma volume) and hemostatic (blood viscosity, fibrinogen levels) parameters as a function of time of day in adult and aged rats. These studies may lead to medical and behavioral interventions for prevention of adverse morning cardiovascular events in older persons.